The soybean, Glycine max (L.) Merril (Glycine max or soybean), is one of the major economic crops grown worldwide as a primary source of vegetable oil and protein (Sinclair and Backman, Compendium of Soybean Diseases, 3rd Ed. APS Press, St. Paul, Minn., p. 106. (1989)). The growing demand for low cholesterol and high fiber diets has also increased soybean's importance as a health food.
Prior to 1940, soybean cultivars were either direct releases of introductions brought from Asia or pure line selections from genetically diverse plant introductions. The soybean plant was primarily used as a hay crop in the early part of the 19th century. Only a few introductions were large-seeded types useful for feed grain and oil production. From the mid 1930's through the 1960's, gains in soybean seed yields were achieved by changing the breeding method from evaluation and selection of introduced germplasm to crossing elite by elite lines. The continuous cycle of cross hybridizing the elite strains selected from the progenies of previous crosses resulted in the modem day cultivars.
Over 10,000 soybean strains have now been introduced into the United States since the early 1900's (Bernard et al., United States National Gennplasm Collections. In: L. D. Hil (ed.), World Soybean Research, pp. 286–289. Interstate Printers and Publ., Danville, Ill. (1976)). A limited number of those introductions form the genetic base of cultivars developed from the hybridization and selection programs (Johnson and Bernard, The Soybean, Norman Ed., Academic Press, N.Y., pp. 1–73 (1963)). For example, in a survey conducted by Specht and Williams, Genetic Contributions, Fehr eds. American Soil Association, Wisconsin, pp. 49–73 (1984), for the 136 cultivars released from 1939 to 1989, only 16 different introductions were the source of cytoplasm for 121 of that 136. Certain soybean strains are sensitive to one or more pathogens. One economically important pathogen is SCN.
SCN accounts for roughly 40% of the total disease in soybean and can result in significant yield losses (up to 90%). SCN is the most destructive pest of soybean to date and accounts for an estimated yield loss of up to $809 million dollars annually. Currently, the most cost effective control measures are crop rotation and the use of host plant resistance. While breeders have successfully developed SCN resistant soybean lines, breeding is both difficult and time consuming due to the complex and polygenic nature of resistance. The resistance is often race specific and does not provide stability over time due to changing SCN populations in the field. In addition, many of the resistant soybean varieties carry a significant yield penalty when grown in the absence of SCN.
SCN, Heterodera glycines Ichinohe, was identified on soybeans in the United States in 1954 at Castle Hayne, N.C. Winstead, et al., Plant Dis. Rep. 39:9–11 (1955). Since its discovery the SCN has been recognized as one of the most destructive pests in soybean. It has been reported in nearly all states in which soybeans are grown, and it causes major production problems in several states, being particularly destructive in the Midwestern states. See generally: Caldwell, et al., Agron. J. 52:635–636 (1960); Rao-Arelli and Anand, Crop. Sci. 28:650–652, (1988); Baltazar and Mansur, Soybean Genet. Newsl. 19:120–122 (1992); Concibido, et al., Crop. Sci., (1993). For example, sensitive soybean cultivars had 5.7–35.8% lower seed yields than did resistant cultivars on SCN race-3 infested sites in Iowa. (Niblack and Norton, Plant Dis. 76:943–948 (1992)).
Shortly after the discovery of SCN in the United States, sources of SCN resistance were identified (Ross and Brim, Plant Dis. Rep. 41:923–924 (1957)). Some lines such as Peking and Plant Introduction (PI) PI88788, were quickly incorporated into breeding programs. Peking became widely used as a source of resistance due to its lack of agronomically undesirable traits, with Pickett as the first SCN resistant cultivar released (Brim and Ross, Crop Sci. 6:305 (1966)). The recognition that certain SCN resistant populations could overcome resistant cultivars lead to an extensive screen for additional sources of SCN resistance. PI88788 emerged as a popular source of race 3 and 4 resistance even though it had a cyst index greater than 10% (but less than 20%) against race 4, and Peking and its derivatives emerged as a popular source for races 1 and 3. PI437654 was subsequently identified as having resistance to all known races and its SCN resistance was backcrossed into Forrest. Currently there are more than 130 PIs known to have SCN resistance.
SCN race 3 is considered to be the prominent race in the Midwestern soybean producing states. Considerable effort has been devoted to the genetics and breeding for resistance to race 3. While both Peking and PI88788 are resistant to SCN race 3, classical genetics studies suggest that they harbor different genes for race 3 resistance (Rao-Arelli and Anand, Crop Sci. 28:650–652 (1988)). Crosses between PI88788(R) and Essex(S) segregate 9(R): 55(S) in the F2 population and 1(R): 26(Seg): 37(S) families in the F3 generation, suggesting that resistance to race 3 in PI88788 is conditioned by one recessive and two dominant genes, where as Peking and PI90763 resistance is conditioned by one dominant and two recessive genes. Based on reciprocal crosses, Peking, Forrest, and PI90763 have genes in common for resistance to SCN race 3 (Rao-Arelli and Anand, Crop Sci., 28:650–652 (1988)). A cross between Peking and PI88788 segregates 13(R):3(S) in the F2 generation, indicating a major difference between the parents for race 3 resistance. Generation mean analysis based on four crosses between resistant and sensitive genotypes; A20 (R), Jack (R), Cordell (R) and A2234 (S), suggests that an additive genetic model is sufficient to explain most of the genetic variation of race 3 SCN resistance in each cross, while the analysis of the pooled data indicates the presence of dominant effects as well (Mansur, Carriquiry and Roa-Arelli, Crop Sci. 33:1249–1253 (1993)). This analysis further indicates that race 3 resistance is probably under the genetic control of three, but not more than four genes.
RFLP analysis of segregating populations between resistant and sensitive lines; PI209332 (R), PI90763 (R), PI88788 (R), Peking (R) and Evan (S), identified a major SCN resistance QTL (rhg1) which maps to linkage group G (Concibido et al., Theor Appl. Genet. 93:234–241 (1996)). In this study, rhg1 explains 51.4% of the phenotypic variation in PI209322, 52.7% of the variation in PI90763, 40.0% of the variation in PI88788 and 28.1% of the variation in Peking. This major resistance QTL was assumed be one and the same in all of the mapping populations employed. However, as pointed out by the authors, it is possible that the genomic interval contains distinct but tightly linked QTLs. In a related study using PI209332 as the source of resistance, Concibido et al., Crop Sci. 36:1643–1650 (1996), show that a QTh on linkage group G (rhg1) is effective against the three SCN races tested, explaining 35% of the phenotypic variation to race 1, 50% of the variation to race 3, and 54% of the variation to race 6. In addition to the major QTL on linkage group G, 4 other QTLs mapping to linkage groups D, J, L and K were identified, with some of the resistance loci behaving in a race specific manner.
Concibido et al. (Crop Sci. 37:258–264 (1997)) found significant association of marker C006V to a major QTL on linkage group G (rhg1) and resistance to race 1, race 3 and race 6, in Peking and PI90763 (Evan X Peking, Evan X PI90763) and races 3 and 6 in PI88788 (Evan X PI88788), in agreement with the previous study based on the P209332 source of resistance (Concibido et al., Crop Sci. 36:1643–1650 (1996)). The resistance locus near C006V was effective against all races tested in all of the resistance sources. While statistically significant against all races, this locus accounts for different proportions of the total phenotypic variation with the races tested. For example, in PI90763 the resistance locus near C006V explains more than three times the phenotypic variation against race 1 than against race 3. The variability can be attributed to differences in the genetic backgrounds, variability among the SCN populations or may be a reflection of the limited size of the plant populations which were employed. This study further identified three additional independent SCN resistance QTLs; one near the RFLP marker A378H mapping to the opposite end of linkage group G from C006V (rhg1), one near the marker B032V-1 on linkage group J and a third linked to A280Hae-1 on linkage group N. Comparisons between the different SCN races indicated that some of the putative SCN QTLs behave in a race specific manner.
PI437654 was identified as having resistance to all known races. Based on analysis of 328 recombinant inbreed lines (RIL) derived from a cross between PI437654 and BSR101, Webb reported six QTLs associated with SCN resistance on linkage groups A2, C1, G, M, L25 and L26 (U.S. Pat. No. 5,491,081). An allele on linkage group G, presumed to be rhg1, is involved with certain SCN races tested (races 1, 2, 3, 5 and 14), and has the largest reported phenotypic effect on resistance to every race. In contrast, the QTLs on linkage groups A2, C1, M, L25 and L26 act in a race specific manner. The QTL on linkage group L25 was reportedly involved with four of the five races, while the QTLs on linkage groups, A2, C1 and L26 were each involved in resistance to two of the five races (U.S. Pat. No. 5,491,081). Webb further reports data that the resistance to any of the five races is likely to result from the combined effects of the QTL involved in each race (U.S. Pat. No. 5,491,081).
Qui et al. (Theor Appl Genet 98:356–364 (1999)) screened 200 F2:3 families derived from a cross between Peking and Essex and identified RFLP markers which are associated with SCN resistance QTLs on linkage groups B, E, I and H. The three QTLs on linkage groups B, E and H jointly account for 57.7% of the phenotypic variation to race 1, the QTLs on linkage groups H and B account for 21.4% of the variation to race 3, while the QTLs on linkage groups I and E are associated with resistance to race 5 accounting for 14.0% of the phenotypic variation. In contrast to previous mapping studies which use Peking as the source of resistance, no significant association was detected to the rhg1 locus on linkage group G. The authors point out that the marker Bng122, which has been shown to have significant linkage to rhg1, is not polymorphic in the population employed (Concibido et al., Crop Sci. 36:1643–1650 (1996)).
It has been reported that the rhg1 locus on linkage group G is necessary for the development of resistance to any of the SCN races. There have been efforts to develop molecular markers to identify breeding lines harboring the rhg1 SCN resistant allele. One of the most commonly used markers for marker assisted selection (MAS) of rhg1 is an SSR locus that co-segregates and maps roughly 0.4 cM from rhg1. This SRR marker, BARC-Satt—309 is able to distinguish most, if not all, of the SCN sensitive genotypes from those harboring rhg1 from important sources of resistance such as Peking and PI437654. Two simple sequence repeat markers have been reported that can be used to select for SCN resistance at the rhg1 locus (Concibido et al., Theor Appl Genet 99: 811–818 (1999)). Satt—309 was also effective in distinguishing SCN resistant sources PI88788 and PI209332 in many, but not all, sensitive genotypes. In particular, Satt—309 can not be used for MAS in populations developed from “typical” southern US cultivars (e.g., Lee, Bragg and Essex) crossed with resistance sources PI88788 or PI209332.
Matson and Williams have reported a dominant SCN resistance locus, Rhg4, which is tightly linked to the ‘i’ locus on linkage group A2 (Matson and Williams, Crop Sci. 5:447 (1965)). The QTL reported by Webb on linkage group A2 maps near the ‘i’ locus and is considered to be Rhg4 (U.S. Pat. No. 5,491,081). Webb concludes that only two loci on linkage groups A2 (Rhg4) and G (rhg1) explain the genetic variation to race 3.